The invention relates more particularly to an installation for thermal conditioning, in series production, of cold preforms of thermoplastic material intended to be transformed into finished containers by forming, each preform having a vertical main axis that goes through a neck, the installation having:                at least one element for holding a preform by its neck in a thermal conditioning zone, the thermal conditioning zone being divided vertically into a first heating part that is intended to accommodate a body of the preform, the heating part being exposed to at least one element for heating the body of the preform, and a second cooling part that is intended to accommodate the neck of the preform;        at least one cooling device for at least one portion of the surface of the preform by producing a gas flow, in particular air, which passes through the thermal conditioning zone.        
The preforms are generally obtained by injection of a thermoplastic material. Then, they are cooled and stored for their subsequent transformation into finished containers. Obtaining the finished container is performed during a forming operation, for example by blow molding or by stretch blow molding, of the walls of the preform.
The preforms traditionally have a body that is intended to be deformed during the forming operation, and a neck that is already molded to its final shape. It is therefore important to protect the neck of the preform so that it is neither deformed nor damaged during the production of the finished container.
To do this, the preform is thermally conditioned just before the forming operation by passing into the thermal conditioning installation. In this installation, the body of the preform is rendered malleable by heating the thermoplastic material that constitutes its walls beyond a glass transition temperature, while the neck is kept at a temperature that is lower than said glass transition temperature to avoid any deformation of the neck.
For this purpose, the thermal conditioning installation is designed so that the preform portions to be heated, i.e., the body, are received in a heating part of the installation, while the portions of the preform that must remain “cold” are received in a cooling part of the installation.
Concerning a thermal conditioning installation in large quantities of preforms, the preforms are mounted on a mobile support in such a way as to make possible the routing, generally in a line, of the preforms to a station for forming during their thermal conditioning.
To keep the neck of the preform at a cold temperature, it is known to have a first flow of cold air circulate in the cooling part of the installation. This air flow is generally transverse in relation to the direction of movement of the preforms, and it is oriented at right angles to a main axis of the preforms.
To produce this flow of air, it is well known to use fans, which agitate the air to obtain a turbulent air flow. The turbulent air flow is carried to the cooling part through ducts.
In addition, it is also known to have a second turbulent air flow, separate from the first cold air flow but parallel to it, circulate in the heating part. This makes it possible to guarantee that the body of the preform is heated homogeneously. This also makes it possible not to damage the outer skin of the preform while controlling the temperature gradient in the thickness of the wall of the preform. The temperature of the air of this second turbulent air flow rises rather quickly as a consequence of the ambient heat of the heating part.
It is also known to use a fan to obtain this second turbulent air flow.
To reduce the thermal exchanges between the hot part and the cold part, it is also known to provide rails along the path of the preforms. These rails make it possible to reduce the opening between the two parts of the installation. These two rails are cooled by internal circulation of a heat-transfer liquid.
Nevertheless, it has been observed that the turbulent air flow used to regulate both the temperature of the neck and that of the body does not make it possible to obtain an optimal temperature for certain portions of the preform.
A preform thus thermally conditioned actually has a very considerable temperature gradient, particularly in a segment of the body located against the neck. Such a segment will hereafter be called “under-neck segment.”
Thus, a marginal part of the first turbulent air flow, used to cool the neck, goes through the space reserved between the two rails to cool down the temperature slightly of the under-neck segment. The result is that the stretching of this under-neck segment must be treated specifically during the forming operation, particularly by controlling a stretching rod. This causes difficulties of adjustment during the forming operation.
Likewise, the second air flow has as its object to homogenize the temperature of the body. Nevertheless, the preform has, at its closed end, an injection point that is made of the thermoplastic material in a low stretch state. It would therefore be advantageous to be able to heat the body portion located in the direct vicinity of this injection point to facilitate its stretching during the forming step.
Moreover, the fans and the ducts used to guide the turbulent air flow are extremely bulky. Such ventilation equipment can sometimes occupy more than half of the space for the thermal conditioning installation.